Autonomous rotor speed restoration and stabilization after helicopter engine failure

Following helicopter engine failure, rapid rotor speed recovery and maintenance of a stable steady-state autorotation condition are essential. To achieve autonomous control of this process, this study first analyzed the collective pitch control characteristics based on sweep-frequency flight test data from a light helicopter in steady-state autorotation. The results revealed a significant phase lag in the rotor speed response to collective pitch inputs, demonstrating that conventional rotor speed feedback strategies carry an inherent risk of inducing system oscillations. Second, based on blade element theory analysis, it was identified that conventional rotor speed feedback strategies lack explicit regulation of the airflow angle of attack. Building on this insight, a vertical velocity feedback strategy was proposed, along with a composite control architecture integrating inner-loop vertical velocity feedback and outer-loop rotor speed feedback. The effectiveness of this composite architecture was further verified from a transfer function perspective. Subsequently, a flight dynamics model applicable to engine failure scenarios was established for the UH-1H helicopter, incorporating rotor disk modeling and dynamic inflow theory. Simulation analysis of the collective pitch response in steady-state autorotation validated the characteristic reverse initial response and phase lag of the rotor speed for the studied helicopter. Finally, simulations for steady-state autorotation entry and disturbance rejection were conducted based on the developed model, comparing the performance of four control strategies: open-loop collective reduction, conventional rotor speed feedback, the proposed vertical velocity feedback, and the dual-loop feedback strategy. The results indicate that the proposed dual-loop feedback strategy overcomes the inherent oscillatory limitations of rotor speed feedback, significantly accelerates rotor kinetic energy recovery compared to single-loop vertical velocity feedback, and provides disturbance rejection capability absent in open-loop control. This study offers new insights for the design of autonomous controllers in autorotation descent and the reliable establishment of steady-state autorotation.